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Can prop wash from a live engine contribute a sideways force on the aircraft?
A recent response to this question, it was posited in the accepted answer that: the rudder "also creates a side force" (as well as a torque) that is countered by banking the aircraft, allowing it to fly straight.
While the answer did indeed explain the control inputs and positioning of the aircraft for lowest drag with an engine out, it did leave me wondering if other forces were in play.
Specificly, any force acting on the center of gravity will tend to move the mass in that direction (unless opposed by another force).  Forces away from the center of gravity tend to produce rotation from their torque.
Realizing there may be points in between where both "push" and torque occur, is the possible that another force, from the propwash of the live engine, is also responsible for creating additional sideways force on the aircraft during an engine out, or is it only from the rudder input used to control the thrust asymmetry?

@quietflyer  the yaw string, if the plane rolls (without turning) but has an AoA, won't be straight back.  Just thinking about what you were saying:  there is some side force on the fuselage that bank must compensate.  In order to staighten a yaw string on a plane that is rolled and pitched, a little more rudder is needed, which actually puts the plane into a slight skid.

I think that the spiralling propwash definitely can contribute a yaw torque by striking the vertical fin and by definition this must involve some sideforce, just wondering how significant it would be on twin/multi engine aircraft, wondering if the question might be improved by asking that more specifically, just a thought.  It looks your comment above is getting at something else maybe, the fact that any rolling motion around the aircraft's longitudinal axis automatically converts angle-of-attack into sideslip.  My hunch is that unless we are talking about rather rapid roll rate, this

My hunch is that unless we are talking about rather rapid roll rate, this doesn't usually play a very significant roll in aircraft's slip/skid dynamics, but it is something to consider--

This is a good question that stimulates a rather interesting analysis--

On a twin with a single fin and rudder, the only significant propeller wash factor is propeller P Factor, as it offsets the thrust line to the right of the propeller axis (for a clockwise-from-behind propeller).
So the right engine's net thrust line is farther away from the airplane's longitudinal axis than the left's, and more rudder is required to counter the thrust offset with the longer arm, all else being equal, which is why losing the left engine (the critical one) is a bigger deal than losing the right (along with the fact that a lot of twins had a single hydraulic pump powered off the left engine, so if you lost the left side, you had to manually retract the gear with a hand pump AND deal with the control issues, all while doing the engine-out drill - a pretty intense exercise).  Of course if you have counter-rotating propellers, conditions are the same with either engine out, especially if you had dual hydraulic pumps or an electric backup pump instead of a hand one.
The spiral wake of the prop is of little significance because the wakes generally miss the fin/rudder, being off to the side.
In a plane like a Beech 18 with two fins/rudders that are directly in the propeller slipstream, you have improved rudder effectiveness because the surface is in the prop blast, and the spiral slipstream may apply its own lateral component to the fin/rudder depending on where the aerodynamic center of the surface is within the spiral flow field, that determines what the overall angle of attack of the surface is.  In any case, it may help or hinder the rudder, but mostly the effect will be lost in the noise of all the other effects.
All that is going on is, the thrust line is offset, which wants to take the plane around in a flat turn, rudder is applied to stop the turn, and you end up with a resultant thrust vector, the sum of the engine's thrust and the rudder's lateral lift, slewed to one side, and the bank applied reorients the thrust vector back to being straight ahead.
The rule of thumb is 5 degrees of bank because most twins don't have yaw strings and the ball doesn't help you much, and 5 degrees is a good ballpark number that works well enough when at blueline speed.  If you had a yaw string, you would just apply whatever bank it takes to align the yaw string, 4 degrees of bank, 6 degrees of bank, whatever.
Imagine taking off in a glider with the tow line connected to the wing leading edge a few feet from the fuselage.   You would have tons of rudder in just trying to keep the nose pointed at the tug, but you would still slew way out to the side opposite to the tow line connection.  You would have to lower the wing with the tow line connected, and at some angle, you would find the glider lined up behind the tug, with the yaw string straight, with the wings banked 5 or 7 or 10 degrees into the side that has the two rope connected.

"The spiral wake of the prop is of little significance because the wakes generally miss the fin/rudder, being off to the side." -- no doubt true-- but the question didn't really address significance --

Significance is implied in the context.  Otherwise, why bother asking?!  ;)

Forces away from the center of gravity tend to produce rotation from their torque.

While technically true, I feel that this sentence can be a little misleading. If you push on the side of a body (such as an airplane), assuming there are no other forces acting on it, the body will both rotate around its center of mass, and move sideways.  In other words, there is always going to be a "push" as you called it. Now, the ratio of "push" to "rotation" will slide more toward rotation the further out you go from the center of gravity, but "push" will never reach zero.
To directly answer your question, no, there wouldn't be any noticeable sideways aerodynamic force on the fuselage in the scenario described. The prop wash from the good engine might contribute some sideways force, but it could be to either side depending on which way the engine is spinning. There might also be some small momentary forces due to the pilot being human and therefore not able to fly perfectly coordinated, but they generally won't favor one side or the other. The bank needed is almost entirely due to the sideways force generated by the rudder.
I think it might be helpful to look at a helicopter in normal flight. The main rotor generates a torque, which needs to be countered by the tail rotor out at the end of its boom. But the tail rotor also generates a sideways force, which needs to be countered by the main rotor.

It was the comparison to helicopters in the referenced question that got me interested.  I wonder if a clever 'copter designer would make the rotor a little high to generate a bit of roll torque.  Having to tilt the rotor a bit (the other way) may lessen the chances of forming a vortex ring above the rotor. +1 for your "push and torque" real world analysis.

Can prop wash from a live engine contribute a sideways force on the
aircraft?

Absolutely.
For a twin-engine aircraft with wing-mounted engines with propellers:
If the failed engine is the "critical engine", then any sideforce and yaw torque from the spiralling slipstream will increase the yaw torque and sideforce that the rudder must generate to counteract the yaw torque from the thrust asymmetry, but will also contribute a sideforce that very slightly decreases1 the amount of deflection of the slip-skid ball associated with zero-sideslip2 flight.
If the failed engine is not the "critical engine", then any sideforce and yaw torque from the spiralling slipstream will decrease the yaw torque and sideforce that the rudder must generate  to counteract the yaw torque from the thrust asymmetry, but will also contribute a sideforce that very slightly increases1 the amount of deflection of the slip-skid ball associated with zero-sideslip2 flight.
These effects may not be easily detectable, since the fuselage and vertical fin are not in the core of the propwash in a twin-engine aircraft of this configuration.  This answer does not intend to imply that these subtle effects are the reason that one engine is designated the "critical engine", and the other engine is not.
If both props rotate in the same direction, then if the props rotate inward at the top of their arcs, consider both engines "critical engines", and if the props rotate outward at the top of their arcs, consider neither engines "critical engines".
Footnotes:

The fundamental reason why the spiralling slipstream produces a change in the net sideforce generated, in the zero-sideslip condition, by the fuselage, fin, and deflected rudder combined, rather than simply being cancelled out by an equivalent sideforce in the opposite direction contributed by the rudder itself, is that the rudder acts at a longer moment-arm from the CG than does combined effect of the rear portion of the fuselage and the vertical fin, and thus the rudder requires less sideforce to create an equivalent yaw torque.

For the purpose of this answer, consider the definition of "zero sideslip" to be that the fuselage is aligned with the free-stream (i.e. unaffected by the propwash) "relative wind", rather than yawed to point to the left or right of the direction of the incoming free-stream "relative wind".

This answer is based on the ideas expressed in this related ASE answer.

@RobertDiGiovanni -- see footnote 1. That's really it in a nutshell, not sure any more need be said.

I think the paragraph about counter-rotating propellers and critical engine is reversed -- outward rotating is both critical (e.g. P-38), inward rotating is no critical (e.g., Beech Duchess).

@FredLarson -- the definition of "critical engine" is "the good engine-- the one you'd least prefer to lose".  Thinking of it that way, do you still think the answer contains an error?  PS my understand is that in the production P-38, both engines rotated inwards at the top of the prop arc, not the other way.

Yes, I do. For example, if the engines rotate outwards, the left engine will have right-turning tendencies. If the right engine fails, these right-turning tendencies will exacerbate the off-center thrust yaw toward the dead right engine. Same thing in reverse for the left engine failing, so both engines are critical. Correct?

@FredLarson -- in the situation described in your last comment, you describe the turning tendencies correctly-- but that means that each engine is turning in the least unfavorable direction for single-engine operation.  The definition of "critical engine" is the engine that is turning in the most favorable direction for single-engine operation-- that's why you'd prefer to lose the other (non-critical) engine.

See https://aviation.stackexchange.com/a/49730/520: "Aircraft which have counter-rotating propellers rotating toward the cockpit on the top side (such as the Beechcraft Duchess) do not have a critical engine, while both engines are critical on aircraft with counter-rotating propellers turning away from the cockpit. The Lockheed P-38 was an example of the latter."

"A/c which have counter-rotating propellers rotating toward the cockpit on the top ...do not have a critical engine, '-- because you are in an equally bad situation if you lose either engine; you'd be better off if your remaining engine rotated the other way.  "while both engines are critical on aircraft with counter-rotating propellers turning away from the cockpit."-- because if you lose an engine, the other remaining engine is turning in the most favorable direction; if you lost an engine  in a twin with both engines turning in the same way, you'd prefer to be in this same situation also.

@FredLarson -- see previous comment, also -- "PS my understand is that in the production P-38, both engines rotated inwards at the top of the prop arc, not the other way."-- sorry, had it backwards.  Both props rotated outwards at the top of their arcs-- the most unfavorable configuration for single-engine flight in terms of P-factor-- yet technically speaking this meant that neither engine would be considered a critical engine.  In a conventional twin, the critical engine is the engine w/ the prop rotating inwards at the top, because that's the engine that you'd least prefer to lose.


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Does thrust vectoring create an additional pitching moment by changing mass flow direction?
Thrust vectoring changes the direction of exhaust gasses to provide force in the direction the nozzle is pointing.  Going back to the ancient Greek steam jet, the aeolipile,  building pressure inside a chamber and releasing it through a nozzle will create a force (action/reaction) in the opposite direction, based on the principles of momentum.
But, the mass flow changes direction in the curved nozzle.  Will this not produce an additional pitching moment in the opposite direction of the curve?
Would this not have to be applied to over-all pitching moment calculations for an engine mount?
The curiosity stems from this recent question about an aircraft with a dorsal single jet engine mount.

Isn't it the point of thrust vectoring? To add pitch (and sometimes yaw and row as well) authority when the elevator isn't very efficient (or nonexistent, like a rocket)?

To answer your question directly, no it will not product a pitch moment (torque) in the opposite direction of the curve, but a force towards the inside the curve. That force can produce a torque on any direction depending on where your center of gravity is.

Are they better off doing all that flow bending on the air intakes?  Why not 1 on top and 2 on the sides (where form drag mitigation is most effective), keeping the (further aft and faster flow rate) exhaust tubing straight?

Engineering is all about compromises. The size of this plane doesn't allow for a L-1011 style S-shape intake and would make the plane too tail-heavy. While the DC-10 style mounts the nozzle too high. Twin engine or even triple engine would need two or three smaller engines which may not be available at this thrust class and would naturally be more complex and expensive. And to be honest, more engines around the tail means higher chance of damaging the tail, should one of the engines blows up.

@user3528438. Not multiple engines, just multiple intakes.  Straight pipes seem to help motorcycles.  Something about less back pressure.  Intakes could be designed in XS so the (curving) airflow is nice and slow.

Seems to me that force (and torque) produced by bend in duct is in same direction as force and torque from directed exhaust nozzle, not opposite.

No, the change in mass flow direction does not cause an additional pithing moment. The mass flow changes direction, and creates forces that will have to be absorbed by the internal support structure of the exhaust nozzle, like the aileron hinges must be supported by an appropriate construction. The mass flow change force is an internal force.
So the dynamics of the mass flow turning a corner are not of consequence to the aeroplane flight path, kind of like a pilot pressing on the side of the cockpit also does not alter the course of the plane.

But if that pilot ejects, what then?

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Will this helicopter fly?
There is a helicopter that can lift 1000 kg.
I'm going to suspend a very large 250 kg square piece of plywood underneath it, wider than the rotors.
On the first try it will be attached directly to the skids.
On the second try, it will be 20 feet below the skids.
Will it fly?  Which one flys better.  Why?

You’ll survive longer in the first scenario.   If you even manage to get off the ground in the second scenario and start moving that plywood will swing up behind and I predict a swift and expensive end to that helicopter.

Pic source
The first try may not result in a lift-off, the second try has a much higher probability of being able to fly.

Large piece of plywood directly attached to the skids: the rotor downwash blows down on the plywood, thereby negating the rotor thrust. Like a person in a cockpit pushing on the sidewalls, or a person in a sailing boat blowing air into the main sail with an air blower: no external thrust is generated. That is, if the plywood is flat.
Plywood attached to 20 ft rope: Now the helicopter can lift off at least 20 ft. And then again the rotor downwash will be blowing on the helicopter  structure itself, but this time a larger part of the downwash may actually be free to bypass the plywood and create useful thrust. In the pic above on the right hand side it may be seen that some of the outside downwash still has some vertical lift vectorial components.

It’s reasonable to assume that once the plywood leaves the ground there will be significant turbulence and buffeting under it.   I’d be surprised if it didn’t oscillate with considerable energy.

@Koyovis maybe the better question would be if a fan blew into a sail, which way would the boat go.  If one removed the big "airmover"  blades and put in, for example, Daedelus props, I think the fan would win.

I second what @Frog wrote, I bet it will end badly.

@Jpe61 yeah probably.

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Who determines the fuel load?
In the commercial side, who determines the fuel load required for any given flight?
The PIC/Captain? Company/Airline? FBO/Airport?

Disclaimer: this answer applies to airlines (operating under 14 CFR 121) but does not encompass all of commercial aviation.
The dispatcher that is working your flight plans the routing and determines the minimum fuel required from takeoff to destination to furthest alternate with additional fuel as required by regulation you operate under.  On top of this, contingency fuel is added which can account for enroute diversions, cruising at a different altitude, un-forecast winds, holding, etc.  The total of this fuel is the "release fuel" and you cannot takeoff below this number.  On top of this, extra fuel will be added for taxi time.
Ultimately, it is the captain's responsibility to decide how much fuel is needed.  For slight increases, airline policy may allow captain's discretion to add fuel.  For larger increases in the fuel order you generally need to call the dispatcher and amend the release -- after convincing the dispatcher you need the fuel.  If you decide you need less fuel (e.g. less contingency fuel) you always need to amend the release so you can legally take off.

Don't forget that "commercial" flights encompass more than just 121 carriers.  Many commercial flights don't even have a dispatcher that works at their company.  :)

I know this thread is really old but maybe you will answer. Can you explain to what degree the person calculating fuel takes into account the number of passengers and weight of cargo? Would they say something like "Flight 123 is only half-full today, let's reduce fuel by XXX", or perhaps "Flight 123 has 114 passengers, times the standard weight of X equals Y, so we need Z pounds of fuel today", or perhaps this is not even considered? I am talking about big-airline type operations here. Thanks.

@GregT disptach determines the minimum fuel amount and the release number they provide is a hard minimum to take off with, though you can call your dispatcher and negotiate within legal boundaries. The fuel order they put in takes into account many things and some computer program determines how many pounds of fuel it'll take to takeoff, make an attempt at landing, go land at our alternate and then circle for 45 minutes for the given parameters.  That then becomes our release number, and the dispatcher generally adds a bit more to account for taxi and some contingency fuel.

@GregT The flight planners (and their system) would work out the optimal flight plan and fuel load based on the pax, weight and other parameters like CoG. The recommended fuel load was presented to the pilots in the flight plan and loadsheet. The crew would then amend the fuel uplift based on their own understanding of WX etc. If they wanted more fuel above the loadsheet's tolerances, they would be told they needed a new plan and load sheet, and this would be created by the flight planners. In Australia at least, the captain's discretion was paramount, and not subject to questioning.

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What are the indications that your turbofan engine has "thrown a blade?"
In an airliner, such as an A380, what indications do the pilots have that an engine has thrown a fan blade?
Will that always result in destruction (of the engine)?

Incidentally, the most prominent incident with the A380 actually wasn't a thrown fan blade, but actually a thrown turbine disc. The engine housings are designed to contain a thrown blade, but they won't (and didn't) stop pieces of a turbine disc. The root problem actually wasn't immediately apparent to them due to the flood of warnings they received as a result of many hydraulic and electronic control lines being severed by pieces of the disc that punctured the wing in several places.

Turbofan engines are designed and tested to contain a fan blade failure. However, this is a violent event and will cause serious damage to the engine. The engine and surrounding components are subjected to large forces and vibrations, and the engine can ingest debris from the damage to the fan, causing further damage. There will likely be an investigation, and the engine will probably be scrapped due to the widespread damage to different components.
To the pilots, it will seem like a bird strike, but probably worse. There will be a loud noise as the fan blade separates and collides with the fan case and following blades. The fan will then be out of balance, so heavy vibrations will follow as the shaft loses its rotational energy. The pilots will be able to tell which engine has failed by the loss of thrust, as well as the indications of vibration and failure on the instruments for that engine.
If the fan does not get stuck, it will continue to windmill, which will result in ongoing vibration from that engine.

engineer and pilot here - formerly of JPL. I've seen blade loss tests and any modern high performance engine will violently destroy itself if a blade is lost. You can see tests of this on youtube for example at that link or by searching on "jet engine blade-out test" (without the quotes). (User fooot gave a longer example video as well.) In these tests they typically use a (very small, prebalanced) explosive charge to separate one blade from the spindle and then the engine vibrates itself to pieces.
Modern engines are constructed near the very limits of the materials used. In fact there is a special joint venture company in the US that makes a lot of the fan blades and which exists just for this purpose. They use special tools to X-ray every blade and measure the center of gravity as the slightest bubble in composites or weight anomaly would be enough to destroy an engine. When a blade is lost, the off center mass causes an engine to vibrate itself violently to death.
In person you can't really hear anything but the vibration and destruction at these tests - I couldn't even hear the explosive charge when it went off (although it seems audible in some of those videos). As fooot noted above, the pilots are informed through the usual engine-out detection methods such as loss of telemetry (and it may start a fire which would be noted and dealt with by fire suppression systems on a larger jet).
Many companies including, I know, Rolls-Royce will invite the public to fan blade out tests to show how their engine bodies control the resulting carnage. It might be a fun afternoon if you're the type to visit aviation stackexchange :) They post videos online for the same reason and you can see a lot of very nice slow-motion videos on the subject.

I think it is very improbable that any engine manufacturer would let Joe Public anywhere near a fan blade off test. Even company employees won't be there unless they have direct involvement in carrying out the test. In any case, waiting around for several hours (or even days) of test preparation for an event that makes a rather loud bang and a few seconds of pretty flames, but with nothing else to see until the high-speed video is available, isn't really "fun." I've been there in an official capacity.

Hi alephzero. I've been to one too, and there were some members of the public (a city councilman and the local news channel among others). I agree it was over fast and we watched from over 100 yards away. I admit that I can't seem to find any mention of public invitations with a quick google search, and I went in the early nineties, is that really over 20 years ago now? Wow. When I went I had only the loosest excuse to be there (JPL engineer, working together with somebody working on nozzles at the jet company parent corp)

Attitudes to public relations have changed over time. These days, the last thing you need is for a no-win-no-fee lawyer to use an eyewitness account from a clueless member of the general public as evidence in court that your products are "unsafe". Of course companies sometimes put official videos of successful tests on Youtube, but you won't get to see the failures!

If you lose a "large" blade like a fan blade, or a "heavy" blade like most turbine blades, the engine vibration monitors will instantly go off the scale and the engine control system should shut the engine down automatically. To give some idea of the forces involved, losing a fan blade from a large turbofan will create an force of about 100 tons on the engine, oscillating at a few thousand RPM until the rotor slows down. In a worst case scenario, the aftermath is enough to shake the entire aircraft to the level where walking about in the passenger cabin is difficult. But aircraft have survived that and landed safely, and for long distance flights over water the plane is certified to survive the consequences for up to 3 hours flight time before reaching the nearest land.
On the other hand, if the tip of 10mm long high pressure compressor blade cracks off in the engine core, you might not even know about it until the next time the engine is stripped down for a full inspection.

nice to have force numbers, upvoted!  Also, I agree about difference between turbing blades and fan blades.

The time that an aircraft is certified to fly with an engine out over water varies greatly by aircraft. For some aircraft, it's only 90-120 minute, while for the 777 it's 5.5 hours. I don't think quad-engine aircraft have a limit at all.

Sure, it doesn't make much sense for a short-haul aircraft to be certified for long-haul operations. But there are two separate issues here. Shutting down one engine out of four is less of a problem than one out of two, but the issue of vibration from a failed engine that is windmilling hazarding the aircraft is the same for both. AFAIK the incident which  prompted a change to the airworthiness regulations to cover that scenario was a 747 that broke a fan blade in flight. The plane survived, but the crew seriously considered the option of ditching into the Pacific ocean.

@alephzero: The 1993 Cathay Pacific incident! Can't believe I forgot to tell that story in my answer! As I recall from the board report the vibrations were so bad the pilots couldn't read the instruments (at least briefly).

If I told you everything I know about that incident (and not just from reading the official reports), I would have to shoot you afterwards ;)

@alephzero Are you implying that information related to the investigation of an engine failure on a commercial airliner is classified? That seems extremely unusual (and I used to work in a jet engine testing facility that dealt with all manner of classified engines.)

Provided the engine survived the event, we are looking at all the engine parameters from the cockpit displays. Those indications are different depending on engine manufacturer. N1, N2, EGT, Oil pressure, ITT, engine vibrations indicator (VIB) and source, EPR (...). Just by looking at the numbers you cannot tell for sure if a blade separated. And it all depends also from the compressor stage the blade separated (first, last...) or if it was a fan blade.

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What is a passenger disturbance level?
I was listening to RealATC for a United Flight UAL1074 which declared an emergency needing to return to Washington Reagan National Airport. During the ATC recording, the flight declares an emergency due to a passenger disruption.
ATC asks for something called the Passenger Disturbance Level. The Copilot replies with a Disturbance Level of 2. As I had never heard this asked for before, I was curious as to what the levels are for and what they each mean?

The FAA has categorized disturbances into four levels:

Level 1: Disruptive behavior - suspicious or threatening
    Level 2: Physically abusive behavior
    Level 3: Life-threatening behavior
    Level 4: Attempted or actual breach of the flight deck

These are also ICAO standard classifications from the restricted Doc 9811, according to IATA. I'm not sure who came up with them first. That IATA document also provides sample things that might be considered to be each level.

So if I understand it correctly: Level 1 is an unruly or drunk passenger that is making a ruckus, level 2 is a passenger that's beyond a ruckus and has started to touch or beat passengers or crew, level 3 is a medical emergency, like a heart attack or a seizure, and level 4 is basically when someone tries to hijack the plane. Does that sound about right? I assume that each level is handled differently once the plane has made the emergency landing?

@NateKerkhofs: Passenger Disturbance Level 3 might cause a medical emergency, but most medical emergencies have other causes.

Level 3 seems more like someone pointing a gun at another person onboard. That also seems a reasonable curve-fit given the other levels listed, and IATA seems to agree; examples given are The threat, display or use of a weapon and Physical or sexual assault with intent to injure.

@NateKerkhofs a medical emergency is not a passenger disturbance and is responded to by medics, not police. Level 3 passenger disturbance is physical violence capable of life threatening injury.

My guess is that it has something to do with FAR 121.580 which states

...No person may assault, threaten, intimidate, or interfere with a crewmember in the performance of the crewmember's duties aboard an aircraft being operated under this part.

The code may be assigned so that the terminal can send appropriate officials to the gate when the plane arrives.
This interesting article breaks it down into 3 "categories" (page 7 table 3) which may be what the pilot was referencing. Looks like this was written in 1997 and may have become law or influenced common standards since then but thats all I can find that seems to be similar to the situation you present.


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What is a "cross check"?
Shortly before take-off, and again before landing, the cockpit (no way to tell whether it was the Captain or First Officer) came on over the speakers, stating:

Flight Attendant's, prepare for take-off/landing and cross-check

What is a cross-check?

I've only heard this (outside the cockpit, instrument cross-checks happen all the time) as part of "arm slides and cross check", same thing?

@falstro I would assume so -- "Doors locked, slides armed" seems like the most obvious thing that Flight Attendants would need to cross-check.

In France, they say "armement des toboggans, vérification de la porte opposées", which could be translate as "arming slides, opposite door (cross-)check"

This website has the definition:

Crosscheck is a generic term used by pilots and flight attendants
  meaning that one person has verified the task of another. In the
  cabin, flight attendants crosscheck one another’s stations to make
  sure the doors are armed or disarmed as necessary.

Obviously they don't want to unduly alarm (already nervous enough) passengers by talking about arming things and slides and locking/securing doors, so the command is given without specifying what is being crosschecked (or maybe just "doors"). The cabin crew knows what they're supposed to do, and whose work to (cross) check for completion.

If you watch the cabin crew after this announcement is made, you'll see them do stuff with the doors and then peer across the cabin to check that the crew member at the opposite door has done their thing correctly.

@PhilPerry On most of the flights I've been on, they've used the same formula: "Cabin crew doors flight position and cross-check, please."

I've been on many flights where the request is the "Arm doors and cross-check", usually Singapore Airlines.

Most flights I have taken in the EU have used a simple instruction to "arm slides/doors" without any cross-check references.

@jCisco: Southwest does that too, if memory serves.


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Are all flights planned?
Do (GA) pilots necessarily have to plan every flight, or can someone just go up and fly? Please note I don't mean actually 'filing a flight plan', but does every flight need to be carefully plotted out in advance?
I can get in my car and just drive. If I really wanted to, I could point its nose West and drive until I wanted to stop. In the first years of flight, like during the barn-storming era, a pilot with a plane could hop in and fly as long as he had gas, it seems like there is less enjoyment of "flying for flying's sake" and more 400 hours of planning for a 4 hour trip.

Are you talking legal requirements..?

No, not at all, unless that is why there is so much planning, if the legal can't be separated.

When reading about what it takes to gain your PPL, one of the requirements is, or was, a three-point cross country flight. That would, obviously be planned out, because that is WHY you're doing it, or one of the reasons. But once I have my PPL, can't I just 'go fly?'

You didn't specify the jurisdiction that you were asking about, but in the US there is a regulation that covers the minimum that you must do before flying.
Much of it is common sense:

You want to check the weather, because it can impact you much more than it usually does in your car and you don't want to get stuck in a really bad situation.
You want to check the Notices to Airmen (NOTAMs) to make sure that you can use the airport (i.e. it isn't closed, or the runway isn't closed, etc.)
You want to make sure that you have enough fuel, because you can't just pull over at the nearest gas station when you get low.
You want to make sure that the runways are long enough and that the airplane can climb well enough so that you don't crash into something.

That being said, if you are familiar with the area and the airports in the area, you have used them before and are already familiar with the performance numbers for your airplane, then you don't really need to do much more than check the weather/NOTAMs and make sure that your airplane is legal and safe to fly.
Things are a little different than back in the barnstorming days, but then there are fewer accidents too.  :-)

§91.103   Preflight action.
  Each pilot in command shall, before
  beginning a flight, become familiar with all available information
  concerning that flight. This information must include—
(a) For a flight under IFR or a flight not in the vicinity of an
  airport, weather reports and forecasts, fuel requirements,
  alternatives available if the planned flight cannot be completed, and
  any known traffic delays of which the pilot in command has been
  advised by ATC;
(b) For any flight, runway lengths at airports of intended use, and
  the following takeoff and landing distance information:
(1) For civil aircraft for which an approved Airplane or Rotorcraft
  Flight Manual containing takeoff and landing distance data is
  required, the takeoff and landing distance data contained therein; and
(2) For civil aircraft other than those specified in paragraph (b)(1)
  of this section, other reliable information appropriate to the
  aircraft, relating to aircraft performance under expected values of
  airport elevation and runway slope, aircraft gross weight, and wind
  and temperature.

It is not necessary to create a detailed plotted flight plan for every flight. However, a pilot is required to ensure that they are able to safely land the aircraft in case of an emergency (including engine failure) at any time.
This is generally not a problem if the pilot is familiar with the area (such as around their local airport or a frequently flown route). However, if the pilot is intending to fly in an unfamiliar area, they will want to do some research beforehand to find out information about things like:

nearby airfields
airspace designations
terrain (mountains)
obstacles (antennas)
restricted and prohibited areas
temporary flight restrictions (TFRs)
weather forecast

In some jurisdictions, the pilot is also required to carry official charts for their intended area of flight. The rules can't force the pilot to read them, but if anything goes wrong the investigators will check to make sure they're present.
During flight training, students are usually required to create a detailed flight plan for a flight.


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What sort of clearances would be required to land a sub-orbital glider at a major US airport
Now that the Virgin Galactic Space Ship 2 is finally flying, it seems possible to "take it on a tour" to promote the feasibility of point to point sub-orbital flight to large airports near major cities.
My thoughts ranged from air traffic controllers clearing its landing in a similar manner to an emergency landing, which may force a few planes to hold for several minutes in their patterns, to airports simply not wanting craft of this nature around.
Plus, there may be regulatory issues as far as spacecraft of landing at airports at all$^1$, although they could request permission to enter into the airspace in a timely manner like all other aircraft.
$^1$ the Shuttle Orbiter could have done it, and is actually closer in size to the type of ship that could carry airliner sized "batches" of passengers, instead of just 6.

Interestingly, if it's registered as a glider, then presumably it would have priority over (almost) all other airport traffic. This is a great question!

Note: for operational reasons there are talks with airport much before a such landing. I doubt airports with much traffic will allow such landing. Inevitably such spacecraft will stop on runway: they need ground equipment and ground crew (to deorbit they use probably toxic fuels). So, it is not just "few minutes in patterns" for other aircraft. And because weight and no "go-around" I assume FAA and operator will choose airports not in very build-up areas (so it excludes most heavy traffic airports). I assume if it become frequent, FAA will issue rules.

In terms of practicality it would almost certainly be better to have the ship land at military bases rather than commercial airports. If the purpose is to show off the ship to crowds, then it's just as easy to get a crowd at the fence of a military base as at an airport. It's much easier to suspend operations at a military base than an airport. And for most people it's not much further to drive to a military airfield than an airport.

As a simpler question, how are gliders handled at towered airports? Not sure if they are part of regular operations anywhere, but it's a building block towards this question.

Your question is multi-faceted. Starting with the premise that the type of vehicle that Virgin Galactic currently operates is capable of point-to-point suborbital flight. This is probably not a correct assumption, as I will explain later, although avoiding too much detail since this is Aviation not Space Exploration. But the more important part of your question involves the clearances required to land a spaceplane at a commercial airport. In fact there are already some FAA approvals in place for this. In May 2022 the FAA gave approval for Huntsville International Airport in Alabama to be used as a landing site for the Sierra Space Dream Chaser orbital spaceplane. This is separate from the license that Sierra Space will need to obtain from the FAA for these landings, which itself is separate from the Vulcan Centaur launch license. The procedures and clearances for suborbital point-to-point landings at a commercial airport would presumably be similar to those needed for the Dream Chaser landings, so we'll take a look at what those are.
The current plan, or at least the proposal by Sierra Space, is that Dream Chaser will land at Huntsville when returning from ISS (International Space Station) cargo missions. There will be no astronauts on these flights. An Environmental Assessment has been completed, and the FAA is currently working with Sierra Space on their proposal to conduct one landing per year at Huntsville in 2023, 2024, and 2025, two landings in 2026, and three landings in 2027. Although a landing in 2023 is no longer possible, the timeline gives an indication that all parties involved expect that the flight rate for landings at a commercial airport can start out slow then ramp up as experience is gained. Also the number of Dream Chaser flights to ISS will be limited, so I wouldn't take these flight rates to indicate the maximum number of flights that a point-to-point vehicle would be allowed to fly.
Dream Chaser's first flight to space will be a combination test flight and cargo flight to ISS, currently scheduled for 1st quarter 2024. The first flight will presumably land at the Kennedy Space Center in Florida as previously planned. It's not clear when the first Dream Chaser landing at Huntsville would take place. I suspect that decision might not be made until after a successful completion of the first flight.
The FAA documents don’t go into detail about airspace closures, etc, as that will apparently be dealt with in the actual launch licenses.

Under the Proposed Action, the FAA would also issue Letter(s) of Agreement to HSV and Sierra Space to outline notification procedures prior to, during, and after an operation and procedures for issuing a Notice to Airmen.

I would expect there would be some similarity to the arrival of Air Force One, with brief closures of the airport and surrounding airspace. Although the big difference here would be that due to its steep descent Dream Chaser would pass very quickly through Class A airspace (18,000 to 60,000 feet), and it would spend very little time in the Class C airspace surrounding Huntsville International. As both ATC and Sierra Space gain experience with these landings, hopefully disruptions to other traffic would be minimized.
The environmental review goes into quite a bit of detail about the procedures involved, so I won't go into all of the details here other than these highlights:

The Dream Chaser vehicle would remain above controlled airspace for the majority of the overflight of
New Mexico, Texas, Oklahoma, Arkansas, Louisiana, Mississippi, Florida, and Alabama. The reentry vehicle
would descend below 60,000 feet altitude above mean sea level (MSL) approximately 10-20 miles from
HSV prior to landing (15 nm southwest of HSV) and would be operating below 60,000 MSL for about
three to four minutes. The Dream Chaser would slow to subsonic speed at an altitude of approximately 35,000 feet while still flying northeast. The vehicle would turn directly onto an extended straight in final approach portion that commences approximately 8.5 nm from Runway 18L-36R threshold at an altitude of approximately 19,000 feet above MSL.

Dream Chaser landings would take place on the 10,000 foot runway 18L/36R at Huntsville.

Huntsville International Airport and proposed reentry site (Source: FAA)
What is interesting is that this is not the longest runway at Huntsville, 18R/36L is longer at 12,600 feet. Apparently 10,000 feet is long enough for Dream Chaser, and presumably using this runway will be the least disruptive to other aircraft, since this runway will be closed from two hours before the deorbit burn until Dream Chaser is removed from the runway eight hours after landing, a total period of about eleven hours. Even though Dream Chaser uses non-toxic RCS fuel and can be approached by ground crew immediately after landing, it is currently planned to allow eight hours for propellant safing before Dream Chaser can be towed from the runway. I am guessing that this is a very conservative estimate, and that after a few landings the amount of time that Dream Chaser is sitting on the runway after landing will be quite a bit less.
Meanwhile runway 18R/36L will remain in operation for other aircraft during this time, except for a one hour period surrounding the landing, beginning 15 minutes before the deorbit burn until Dream Chaser lands about 45 minutes later, for a total closure of the airport of about one hour. However it seems this will likely only be the case for the first few landings, as elsewhere in the same document it states:

Due to the approach from the west to a landing on the Runway 18L-36R, the operation also has the
potential to affect aviation operations using Runway 18R-36L. Sierra Space’s Concept of Operations would
indicate a time period during which Runway 18R-36L may be unavailable to landing traffic as the re-entry
vehicle transits to the extended Runway 18L-36R centerline. Concurrence with the FAA ATC would be
required to determine the amount of time Sierra Space would need to ensure the approach corridor was
free from conflicting traffic.

This passage would only make sense if the expectation is that after the first few flights runway 18R/36L will remain open during the entire reentry and landing procedure, unless Dream Chaser is on a trajectory that would cause it to pass through or near the 18R/36L approach or departure corridors.
SUBORBITAL POINT-TO-POINT
As mentioned the clearances and procedures for suborbital point-to-point landings would likely be similar to the procedures for the Dream Chaser orbital landings.
I will give an explanation now about my statement that Virgin Galactic is not currently, or for the foreseeable future, capable of operating point-to-point suborbital flights.  I am looking at it from the requirement that sustainable point-to-point rocket travel would have to be a practical alternative to airline travel. For this discussion I am assuming that price is not a factor. Virgin Galactic’s SpaceShipTwo (and upcoming SpaceShip III) are not really up to this task. SpaceShipTwo is released about 25 miles (41 km) from the landing site at Spaceport America in New Mexico, following a one hour climb to the 45,000 ft release altitude while attached to the White Knight Two carrier plane. After release from White Knight Two and igniting its rocket motor, SpaceShipTwo climbs nearly vertically to an altitude of about 55 miles (300,000 feet), then literally falls back down into the atmosphere and glides back to the airport for a landing.
A more horizontal flight path could carry SpaceShipTwo farther, as speculated in this question. However the answer given there of 3,000 km (1,800 miles) seems way too high. My estimate would be that SpaceShipTwo could go perhaps 40 miles downrange during the one minute of powered flight. That's pretty much a guess, but it's based on the assumption that the observed reentry speed of about 2,000 mph is the maximum speed they can fly laterally. They could climb higher and go say 3,000 mph but then that's how fast they would hit the atmosphere on the way down and I tend to doubt that it can take that much heat. 2,000 mph for one minute is about 40 miles. That would be followed by a glide of maybe 75 miles, figuring gliding from 80,000 feet at a 5 to 1 glide ratio (only number I could find). This would be in addition to the 300 miles or so that White Knight Two would carry it downrange prior to the release (observed average speed of 260 knots for one hour, 35 minutes of which is spent climbing to altitude). For a total of about 400 miles.
If that estimate is accurate that is similar to relatively close destinations such as between Los Angeles and San Francisco which are 350 miles apart, and leaves some margin. That's great, however a subsonic airliner can make the same LA to San Francisco flight in an hour and a half, so this would make it more of a novelty flight since the Virgin Galactic flight including the White Knight portion would take almost as long.
Longer suborbital flights would need more rocket power and it would also generate a lot more heat during reentry. There are no known current plans for Virgin Galactic to develop this type of vehicle, even if Richard Branson at various times has stated some grand future plans. And coast to coast or intercontinental point-to-point flights would require essentially orbital class rockets and reentry vehicles. This is why you sometimes hear the SpaceX Starship mentioned as a possible point-to-point vehicle.
But your question about landing at a commercial airport is still relevant even for Virgin Galactic, who could benefit from offering suborbital tourist flights from airports around the country or even around the world. Although more likely these flights would be conducted at regional airports located near major cities that have long enough runways, for example Palmdale Airport which is located 40 miles from Los Angeles.
One noticeable difference between Dream Chaser's orbital reentry compared to Virgin Galactic's suborbital hops, is that VG's coordination with ATC would revolve around the release from White Knight Two, since that starts the clock ticking about 15 minutes prior to landing. This is compared to about 45 minutes prior to landing for a Dream Chaser deorbit burn. Each has its advantages as it relates to coordinating with an airport. The procedure would probably be that the Virgin Galactic pilots would inform ATC of the planned release time and estimated touchdown time. ATC would then approve or deny the request based on the expected traffic situation at the planned landing time. If the request is denied, White Knight Two could simply do another lap in its racetrack pattern at 45,000 feet, and then make another request with updated release and landing times. Although at the less busy airports that Virgin Galactic would likely be conducting the tourist flights out of, I would think that in most cases the initial request would likely be approved, especially since I'm sure Virgin Galactic would be coordinating with the local airports prior to each flight.

Logistically, no, they are not ready for point to point.  Shame they gave up Cosmic Girl.  That 747 could have carried it a long way, with paying passengers to boot.  Yes, the Space Ship would be released for its 15 minute flight, with loiter flexibility (for a landing slot) going with the carrier plane.  Hoping to see Dream Chaser fly soon too.

@RobertDiGiovanni - They have probably pushed the Rutan design and hybrid motor as far as they can go. They seem to have given up on getting to the Karman line, even flying 4+2 passengers instead of 6+2 they can barely make 55 miles. Maybe they will eventually eke it up to 62 miles on occasion. But they took forever to transition from Mojave to New Mexico. And it's taking forever to build the second ship, hard to imagine they will ever complete the planned fleet of five. It is too bad about LauncherOne, at least Cosmic Girl went to Stratolaunch not to the boneyard.

BTW, both Dream Chaser and SpaceShip Two use skids (Dream Chaser two on the MLG, SpaceShip Two one on the nose gear), which also plays a role, as you'd probably need to inspect the runway after the landing.

@JörgWMittag - For sure that will be needed. On page 2-14 of the environmental assessment it says that the runway will be inspected 4 hours before the deorbit burn, inspected again 15 minutes before the deorbit burn, the entire runway will be inspected again 10 minutes after landing, and after Dream Chaser is towed off the runway they will inspect the immediate vicinity around the recovery area. Actually Dream Chaser also has a nose skid and has tires for the main landing gear. Interestingly SpaceShipTwo has a wooden nose skid. I'm not sure what material is used on the Dream Chaser nose skid.

Thanks for the correction. That was probably the X-15 I was thinking of.


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Is silk thread still being used to prevent oil loss from Lycoming engines?
A teardown/rebuild video of a Lycoming O-360 showed the mechanic laying silk thread along a path where 2 engine parts were to be bolted together$^1$.  This would be, in addition to a gasket, to prevent oil leaks.
It seemed possible to have a grooved seat/O-ring type of arrangement as well.
Laying thread is good, but seems to carry the possibility of human inconsistency.  While watching the rebuild video, with the mechanics following aircraft grade specs and tolerances, the part with the thread had me wondering (along with wiring castle nuts instead of using cotter pins).
Time honored, yes.
Could a new mechanic in training expect to be taught the silk thread art for rebuilds?
$^1$ Another source mentions Hylomar or Loctite

You'll do whatever the Lycoming overhaul manual says to do, and the Lyc overhaul manual still calls for silk thread laid into the gasket compound. It says "non hardening gasket compound" which is that Permatex sticky goo stuff that gets on everything when you use it.
It works really well; no need to improve on it.  Hylomar could work (it was developed by Roll Royce for their engines), but I've seen it extrude from the joint in cases where there is potentially a lot of pressure behind it (I tried it as a pad sealant on my own engine and noticed that happening) and if I was assembling an engine I'd stick with the Permatex and silk.

Yes, and LocTite might get too brittle.  Being a research person, I would get one engine, machine a groove or two, and try silicone O rings.  But it seems essentially what the silk thread is so ...

Come to think of it, maybe the engine could carry a few quarts more oil too.

On the cylinder through studs that clamp the bearing races together, Lyc does use O rings at the case seam to keep oil from running down the stud bore. On the earlier O-290 family, they didn't and just depended on interference between the stud and stud bore in the case. My O-290 as a result leaks oil past the cylinder hold down studs/nuts on 3 cylinders. The only fix is to remove the through stud, ream the bore, and install an oversize one with a fresh interference. That can actually be done in the field, but not worth the hassle for a minor oil leak.

I little checking came up with fiberglass reinforced silicone gasket material.  But I would extensively test it for many hours.  The silk thread thing I would learn if it helps.

The problem is the joint is the bearing shell clamping halves, and any clearance between the case mating surfaces has to be carefully controlled. You need the thinnest possible gasket that does the job and can tolerate vibration. Silk thread is ideal, silk being extremely strong..

I could learn it, and Permatex seems to be similar to some coating formulas.  Also looked at soft brass gaskets in motorcycles.


text:
Would a vertical stabilizer that extends both above and below the centerline be benefit or hindrance?
Ignoring for a moment potential schematic difficulties in creating an aircraft that can land and take-off without threatening a downward vertical stabilizer, would a vertical stabilizer that extends both above and below the aircraft's centerline improve or hinder in-flight stability?

Other examples for folding vertical tails: MiG-23 (fin below fuselage for take-off and landing) and Saab-37 Viggen (upper tail to fit into existing bunkers).

The guys over on physics.SE talked about the characteristics of "adverse roll" from yaw (rudder) input - their explanation is also worth a read.

As long as the vertical tail surface sits above the centerline (more precisely the longitudinal axis of inertia - for now we can assume both fall together), any side force will also create a rolling moment.  This is undesirable because now a yaw command will not only create the intended yawing moment, but also a rolling moment.
If you want to yaw in order to turn, the vertical tail above the centerline will even roll the aircraft in the wrong direction, so you need more coordinated aileron command than with a symmetrical vertical tail.
So the short answer to your question is: Terry is right: Extending the vertical tail below is clearly a benefit.
This can be seen by the widespread use of ventral fins in combat aircraft. Adding area below the centerline helps to reduce the roll contribution of the vertical tail in a sideslip, which improves handling characteristics (more directional stability AND lower yaw-induced roll). See the picture below (taken shamelessly from Ray Whitford's Fundamentals of Fighter Design) for an example where the effects of additional vertical tail area are compared to the effects of a ventral fin of the same area.

At supersonic speeds, the low aspect ratio ventral fin has a nearly constant contribution to directional stability, whereas the wing-like vertical tail loses effectivity in proportion to the Prandtl-Glauert factor $\frac{1}{\sqrt{Ma^2-1}}$. The small ventral fin of the F-104 raised directional stability by 30% at Mach 2.

At high angles of attack, the vertical tail is in the wake of the fuselage, where air density is much lower than on the bottom at supersonic speed, which reduces local dynamic pressure and, consequently, effectivity. The ventral fin then is in ideal flow conditions, so it can help to stabilize the aircraft at high angle of attack, just when the fuselage contribution to instability is largest.

but having a rolling moment with the rudder will help when over-rolled in severe turbulence

Yes, there are of course cases where the top-heavy rudder is a benefit. But overall a neutral rudder would be best, wouldn't it?

The "good" roll comes from dihedral or sweep, and is due to the sideslip that will happen anyway, regardless of the vertical position of your rudder.

Not to suggest this as an answer but as a minor factor that may be a consideration. When the vertical stab is above the aircraft's center of gravity, its drag causes a slight pitch up force assisting the elevator in supplying the pitch up force needed to counter the typical pitch down tendency due to the center of gravity being just forward of the wing's center of lift. If you put half of the vertical stab below the aircraft's CG, you will cancel this small benefit and cause the elevator to do all the compensating itself thus adding extra (albeit a small amount of) drag.
The drag vector of a horizontal stab in a tee tail configuration offers a bit of help in this regard too and if you placed half the vertical below the centerline on a tee tail aircraft, the lowered horizontal's drag vector arm would reduce the pitch up force supplied by its drag and you would get a slight overall drag increase from the elevator's extra load.


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Why don't all aircraft use/need nacelle chines?
Most modern commercial aircraft (e.g. 737, 767, 777, 787, 747-8, A380, etc.) feature inboard nacelle chines to improve flow over the upper surface of the wing at high angles of attack. However, many other similarly purposed aircraft (e.g. DC-8, L-1011, 757, 747-400) do not have nacelle chines. If these chines are so critical for optimum takeoff and landing performance, how are so many planes able to function just fine without them?

@tpg2114 I would have thought that nacelle chines were literally the cheapest aero modification you could possibly make to an aircraft. They are certainly less involved than a flap redesign or winglets.

For what it's worth, the DC-8s with high-bypass CFM56 engines do have them.

The best answer is given by user2168 in this post

The chines help when the nacelle is very close to the wing. When the engine of Boeing's 737 was changed from the slim JT-8D to the higher bypass ratio CFM-56 on the 737-300 and later, the chines were essential to avoid large flow separation. Generally, aircraft with higher landing gear can afford to have more space between nacelle and wing and do not need fixes like chines. But when a new engine has been fitted to an existing design, chances are that some vortex generators are needed to fix the new flow conditions.

For the nitpickers: The 737-100 and 200 had underslung nacelles which left no space between wing and nacelle, but that is aerodynamically similar to a nacelle at a larger distance, because no narrow flow paths are created which produce steep pressure gradients. Only a closely mounted, but still separate nacelle will have these flow conditions which need to be remedied by energizing the boundary layer.

woah, impressive answer!

So what about the 757? It's engines are in relatively close proximity to the wing, yet has no chines. What gives?

@Bryson S. You are right, but it had the advantage to be designed that way. The 757 was delivered from the start with the PW-2000 or the RB-211, and the nacelles could be optimized together with wing and landing gear in the design phase, a luxury which the 737-300 did not enjoy.

I have no idea why the C-17 needs them. It has even two per nacelle, and with a design which used those engines from the start. Maybe they were added by Marketing.

Also I don't think the A330 or A340-200/300 has nacelle chines either. I think there may be a drag penalty associated with the chines during cruise, which may impact the decision to put them on a given airplane...

@PeterKämpf WRT the C-17: Patent US 6964397 mentions bilateral chines - "In a vortex control device to improve lift, most positive difference is yielded by a chine on an inboard side of the engine nacelle. However, placement of a chine on the inboard side of the engine results in increased drag over the wing assembly. By contrast, the drag reduction benefit is yielded by placing a chine on an outboard side of the engine, although chines on both sides can have positive lift improvement effects."

@sdenham: Thank you for the detail! But apart from lift, they produce drag, even though the patent claims they reduce drag. This they can only do if the basic design was deficient. I still think of them as an embarrassment for the responsible aerodynamicist.

@PeterKämpf The 'background to the invention' section of the patent offers a motivation for their use: reduce the noise footprint by steepening the climb after takeoff. The C-17 designers may have been optimizing for something else, such as minimizing the takeoff distance.


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How do size and speed affect sonic boom strength?
The Boeing 2707 SST program of the 1960's was very ambitious, aiming for 300 passenger aircraft flying at speeds of Mach 2.5 to 3.
The similarly sized XB-70 was test flown supersonicly along a potential commercial route over land, producing "numerous complaints of cracked glass and broken plaster".  Rising fuel prices and political opposition canceled the program before prototypes could be completed.
What would the mathematical relationship be between the intensity of a sonic boom (of the same aircraft at the same altitude) at Mach 1.5 vs 3.0?
Would it be 1/2, 1/4, or be predictable at all?
What would be the sonic boom relationship be if a smaller scale version of 1/2 it's mass flew at the same Mach and altitude?

I had not heard of a test along a commercial route. Sonic boom tests I know of that "targeted" civilians were the 1961 St. Louis tests using B-58 and F-106, and the 1964 Oklahoma City tests using B-58, F-106, F-104, and F-101. The 1964 test resulted in public complaints and negative publicity for the SST program. In 1966 the XB-70 conducted test flights at Dryden as part of the National Sonic Boom Program, according to the Dryden website these tests targeted instrumented buildings as well as "100 test personnel".

A larger and heavier aircraft must displace more air and create more lift to sustain flight, compared with small, light aircraft. Therefore, they will create sonic booms stronger and louder than those of smaller, lighter aircraft. The larger and heavier the aircraft, the stronger the shock waves will be.

Source: NASA Facts, Dryden Flight Research Center

Since the question asks for a mathematical relationship, you might want to add that sonic boom strength grows with the inverse of distance squared and linearly with airplane weight.

@PeterKämpf thanks!  Do you think designing an aircraft asymmetricly like the BV 141 might cause a more diffuse supersonic shock wave?

Airplane shape has no bearing on it whatsoever?

@Sixtyfive It does! Slender aircraft cause weaker booms than blunt aircraft. Just look how long the tip of the X-59 is! But it is harder to express that in a simple mathematical relationship.

@RobertDiGiovanni Interesting idea! I think a slender Sears-Haak-body will yield the best results, and asymmetry will not help. But that is just me guessing ...

@PeterKämpf further study has shown the N wave coalesces from individual shocks as the wave progresses to the ground, so you may be right.  Some (good) work was done modifying the forward portion of an F-5 in 2003 but then they just stopped and put it in a museum.  Disappointing. I wonder (next) how a canoe (or Viking longboat) shaped hull would do, perhaps deflecting the bow and stern waves a bit more off to the side rather than downwards.


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State of the art in aircraft industry for flutter analysis
I'm an aerospace engineering PhD student and for my research I'm investigating how to come up with a way to pick the most critical flight conditions for flutter. The main idea is to understand how it is currently done in industry and expand that approach for nonlinear cases.
I've been scouting the web to find information on how companies do that (Airbus, Boeing, Embraer etc.) but I was only able to find a paper from Dassault (A Review of Industrial Aeroelasticity Practices at Dassault Aviation for Military Aircraft and Business Jets) that briefly describes their practices for flutter, without really answering my question.
FAR 25 (hence same for EASA) in AC 25.629-1B (the "extension" of part 25.629, which is dedicated to flutter) states that:

For nominal conditions without failures, malfunctions, or adverse conditions, freedom
from aeroelastic instability is required to be shown for all combinations of airspeed and
altitude encompassed by the design dive speed (V_D) and design dive Mach number
(M_D) versus altitude envelope enlarged at all points by an increase of 15 percent in
equivalent airspeed at both constant Mach number and constant altitude. The figure below
represents a typical design envelope expanded to the required aeroelastic stability
envelope. Note that some required Mach number and airspeed combinations correspond
to altitudes below standard sea level.

So does anybody know whether designers only pick a few points at the boundaries of the envelope to actually check for flutter? Or something else is done? For example, in the case of the envelope above this would mean running a velocity-based linear flutter search for several mach numbers between C'' and B'.
Thanks everyone!

I think that the "State of the art in aircraft industry for flutter analysis" is proprietary of each company, not sure if you can find something more than simple and generic explanations...

@sophit I am not sure that is the case, because aircraft manufacturers have to document all their testing results and methodology as part of the certification process.

I don't know if those documents are publicly available

Airbus uses Simcenter Testlab to streamline and improve its flutter analysis process. Their flutter testing is divided into three parts:

Real-time: Live data is collected during the flight to check safety
and continue the flight envelope
Near real-time: Rapid modal estimation is used to determine the
flight's overall safety and the flutter test program
Offline: Recorded flight data is analyzed in detail and a final report is
produced

Simcenter Testlab Flutter Analysis is a Siemens Digital Industries product that is part of the Testlab system. There is a case study on the Siemens web site for the product covering how Airbus used it on the A380 program.
(Full disclosure: I used to work for Siemens and I haven't received a dime from them in 42 years.)

Military flutter testing philosophy and practice may be of interest, also. As a former flight sciences manager for high-performance aircraft, we built up to and then flight tested at several 'edge' points, yes. Of course, ground testing and analyses would cover more far more flight conditions and configurations.


text:
What is magnetic variation under RADIO AIDS TO NAVIGATION in FAA Chart Supplement, and How to use it?
I'm going to use couple examples to ask my question:
Looking at KBVY chart supplement, it says:

RADIO AIDS TO NAVIGATION: NOTAM FILE LWM.
  LAWRENCE (L) DME 112.5 LWM Chan 72 N42°44.43 ́ W71°05.69 ́ 140° 12.3 NM to fld. 306/0W.
  DME unusable:
    160°–195° byd 23 NM blo 2,500 ́

Looking at KMLT chart supplement, it says:

RADIO AIDS TO NAVIGATION: NOTAM FILE MLT.
  (H) (H) VOR/DME 117.9 MLT Chan 126 N45°35.20 ́ W68°30.93 ́ 317° 8.0 NM to fld. 553/20W.
  DME unusable:
    330°–350° byd 35 NM blo 6,000 ́
  VOR portion unusable:
    330°–340° byd 35 NM blo 6,000 ́

Referring to FAA Chart Supplement guide:

KBVY is 12.3 NM from KWM-DME on the bearing of 140°. What does 0w variation mean?
KMLT is 8.0 NM from MLT VOR on the bearing of 317°. What does 20W variation mean?
KBVY is close to the middle of 14°W and 15°W isogonic lines, and KMLT is somewhere between 16°W and 16°30'W isogonic lines, closer to 16°.
I know what magnetic variation is, and how to use it to convert true course to magnetic course. What I don't understand is what is it implying here, in the context of chart supplement, and how one would use it, for what purpose?
It clearly doesn't correlate with the isogonic lines in that area, and the bearings given are magnetic so they shouldn't need conversion (see here: What are the units for the bearing to center of an airport from radio aids to navigation in chart supplement?).

Conceptually, VORs (and VORTACs) emit radials which are fixed lines radiating out from the VOR. One of those radials is designated "north" (0º, 360º) and all of the other radials are designated based on their relative position from that "north" radial.
The decision as to which radial is called "north" is 1) completely arbitrary, from a technical standpoint, and 2) unvarying, from a technical standpoint.
Therefore the "north" radial is chosen so that, at the time of installation, it is the radial which points directly to magnetic north. If, at the time of installation, you start at the VOR and follow your magnetic compass "due north," you are walking along the 360º radial of the VOR. The difference between magnetic north and true north is magnetic variation, as you know and expect, but this is not relevant at the moment of installation.
However, the Earth's magnetic poles shift over time. Here is the answer to your question: The radials of a VOR do not shift over time. If you return to the same VOR forty years later, the 360º radial will still be pointing the exact same direction it was when it was installed. But if you follow your magnetic compass "due north" you will no longer be walking along that 360º radial. That is why the variation listed for a VOR does not match what you would expect from the current isogonic lines.
The discussion is further confused because Skyvector, at least, assumes that when you are flying outbound from a VOR you are likely to be navigating using a radial off that VOR—and it silently provides you with a radial instead of a bearing. For example, if you enter into Skyvector the route KMLT..MLT..KMLT it tells you that you would fly a 133º track to go from the airport to the VOR, but on your return you could fly the 317º radial to get to the airport, rather telling you to fly a 313º bearing. (Note that the current variation is 16ºW while the variation for the NAVAID is 20ºW—thus a 4º difference between radial and bearing.) This certainly tripped me up when I was trying to confirm what I said above.

Note that DME stations do not emit radials. Instead they only tell you your distance to the station but nothing about your bearing. As such they cannot be said to have any variation at all; this is depicted in the Chart Supplement as zero variation, although the correct term would probably be "null" or "not applicable."

Thanks @randomhead. Just to make sure I understand this: since LWM is DME only, if I get an old wet compass over LWM and fly the 140° magnetic heading, I’ll get to KBVY. I can do the same thing over MLT with a wet compass, or if I want to use the VOR, I’d have to apply 20W correction to VOR radial: 317-20=296 is the radial I want to fly from MLT. Is that correct?

Sounds about right, but for two things: 1. Skyvector says that LWM to BVY is a 154º magnetic bearing, not 140º (remember the 14º variation is the difference between true and magnetic; you have a magnetic compass so when Skyvector gives the magnetic bearing you don't need to adjust further). And 2. You want to compare the NAVAID variation with the current isogonic line; looking at MLT you would expect to need only a 4º correction, not the full 20º. Although strangely enough Skyvector shows a 317º magnetic bearing from the VOR to the airport, which seems to line up with the actual 317º radial.

@757, if MLT was installed when the variation was 20° and the current variation is 16° then why does flying the MLT 317° radial make you track 317° magnetic bearing? I would expect a 4° difference. That's what I'm confused about.

@757 that all makes sense, except that Skyvector says the waypoint PATTA is a 318° bearing from MLT. But PATTA is canonically on the MLT 318° radial as well. Because MLT's variation does not match the current variation this should not be the case, if my answer is correct. I'm at a loss to explain it.

@757 that makes sense and I think I understand what's making me confused now. Skyvector shows a number associated with each flight leg, and it does not always show the same kind of number. If you make a flight plan KMLT..MLT..KMLT it shows that you would fly bearing 133° to get to the NAVAID, but then it's smart enough to recognize that MLT is a VOR so it tells you to fly the 317° radial to get back to the airport, instead of telling you to fly a 313° bearing. Makes sense once you figure it out but very confusing (at least to me) before then.


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How are the Tupolev Tu-95's blades shaped?
The Tupolev Tu-95 "Bear" seems like an awfully cool piece of innovation with its unusual contra-rotating supersonic-tipped props.  From what I've been reading it sounds like it's been the only mass production airplane to use props at those speeds (there were apparently a handful of American examples as test aircraft but NASA abandoned the project?).
I'm curious what profile of blades they used.  I know there are lots of supersonic-capable airfoil shapes out there, but it's not clear which if any of these were used for the Tu-95 props, or even whether a fixed wing supersonic blade profile is appropriate for a prop.  Any ideas?

Arguably, it was, in a way, something opposite: a safe bet technology chosen in preference to the true innovation: a jet engine (on a long-range aircraft).

The TU-95 propellers are not the only ones whose tips operate at supersonic speeds. The catch is that doing so requires significantly more torque from the engine to overcome the drag from the supersonic tips. That's why the engines on the prototype TU-95's had to be discarded and replaced with more powerful ones when the aircraft entered production. I can't find any source for the airfoil used on those propellers.

@JuanJimenez what are other examples?  The XF-84H is the only other one I know of.

Every turbofan out there uses fan blades whose tips operate at supersonic speeds. The fan blades are just like propellers, except that they are "caged" in a way that overcomes the problems with propellers trying to operate at those tip speeds. Some turboprops can also operate briefly in that range but the problem is that it greatly reduces propeller efficiency and increases metal fatigue.

The XF-84 was quite different: Spinning at 3000 RPM, most of the XF-84 prop blades' was at supersonic speed. The 850 RPM on the Tu-95 actually minimized the supersonic blade section.

See here: https://i.sstatic.net/UhDpb.jpg

@Zeus (I'm aware I'm replying to an old comment) arguably it was a damn good bet, not that many designs from that era are still in active use. The Bear has truly overcome the test of time.

@Jpe61, …. except it's direct counterpart, which first flew earlier in the same year, the B-52!

Slight frame challenge, the tips might not be supersonic in cruise, only at maximum speed: "The propeller diameter is 5.6 m and their speed is 750 RPM, so the circumferential component is 5.6⋅π⋅750/60=220m/s. Add to this the cruise speed of Mach 0.67 (taken from this site — others list quite incredible numbers) at altitude, where the speed of sound is 295 m/s. Mach 0.67 equates there to 197.65 m/s, and vector addition gives 295 m/s for the propeller tips, exactly Mach 1.0. This means that the propeller is subsonic over its whole span."

@Jan Hudec yep, hence not that many designs and not  no designs :)

The propellers were designed by OKB-120 headed by K.N. Zhdanov, type AV-60N.  This type was used on a few other aircraft, but unfortunately I can't find any further details about it.  Can somebody visit one and inspect it?


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Is the 737-800 circular PFD for AOA or RA?
So, I have seen some people on the Internet showing the 738's PFD circular gauge for RA, but others show that the gauge is used for AOA(angle of attack). Which one is true? Is there a switch to change the circular PFD gauge between RA and AOA?

For readers who may not know, RA is Radio Altimeter: https://en.wikipedia.org/wiki/Radar_altimeter

It’s a customer option (i.e. the operator can choose and specify) to switch between the two; presumably by pin programming the computer responsible for generating the display symbols.

So there is a switch in the cockpit or it can only be set during the manufacture of the plane?

@lpydawa Neither. I’m pretty sure it can be changed during maintenance input (if the option to be allowed to do so has been bought from Boeing for the particular aircraft in question). Maybe someone else can confirm?


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Can a 737NG still fly when every single electric run device/system is dead?
So, as my title suggest, I wonder if the 737NG can still fly without any of it's electric run systems, which means anything that runs on electricity is dead.

TLDR: Yes, the 737 can still fly, the gear can be lowered and one can brake after landing.

A complete failure of every single electrical system is extremely unlikely. The QRH (Quick Reference Handbook) does not even mention this case. Usually, at least the battery remains and can power all emergency systems for at least 30 minutes (60 min, if 2 batteries are installed). See also What happens with the avionics if in midair there is loss of power or fuel flow interrupted?.
Even in the unlikely case of a complete electrical failure, the aircraft is still flyable. As long as hydraulic pressure is still available, the flight controls would function normally. But even without any hydraulic pressure the aircraft can be controlled via manual reversion. This means the mechanical link of the flight controls to the hydraulic actuators at the control surfaces can transmit the forces to the control surfaces (similar to what happens when hydraulic power steering fails in your car). The forces required will be substantially higher than usual, but the aircraft is flyable. Also manual pitch trim via the trim wheel in the cockpit is still possible.
Some things to consider:

A complete electrical failure would include a failure of both EECs (Electronic Engine Control) and therefore both engines would be dead. You would have to sail the aircraft down. As Ralph J pointed out in the comments, the engines could still windmill and might still provide some hydraulic pressure via the engine driven pumps, but far from normal pressure.
Normal flaps extension is powered by hydraulic system B. The FCOM says that the flap lever

selects position of flap control valve, directing hydraulic pressure for flap
  drive unit
  position of the LE devices is determined by selecting TE flap position

so this might still work with hydraulic pressure. But note that there are electronic components like the flaps/slats electronics unit (FSEU) involved, which would no longer function without electrical power. I am not entirely sure if that would already prevent normal flap operation. See also the flaps and slats control diagram below.
Flaps can also be extended without hydraulic pressure using the alternate flap extension mechanism, but this is electrically powered, so alternate flap extension would be impossible in this case.
Normal gear extension is powered by hydraulic system A. If the pressure is still sufficient to release the up-locks, the gear can then fall down assisted by gravity and the airflow:

When the LANDING GEAR lever is moved to DN, hydraulic system A pressure
  is used to release the uplocks. The landing gear extends by hydraulic pressure,
  gravity and air loads.

Otherwise, manual gear extension is always possible via three mechanically connected cables in the cockpit using a gravity drop of the gear. It is however not guaranteed that the gear will lock in place.

If hydraulic system A pressure is lost, the manual extension system provides
  another means of landing gear extension. Manual gear releases on the flight deck
  are used to release uplocks that allow the gear to free–fall to the down and locked
  position. The forces that pull the gear down are gravity and air loads.

Without hydraulic pressure, both normal and alternate braking is unavailable. The brake accumulator will however still have sufficient pressure in it for braking after landing.
Older 737 NGs still have mechanical standby instruments (artificial horizon, airspeed indicator, altimeter). These could still function. Modern NGs have replaced this with an electric standby instrument. Without any instruments, it will be extremely difficult to control the aircraft.
All radios and the transponder would be inoperative, so no airport would even know that you are coming to land there.

Without any hydraulic pressure, the following components are inoperative:

All flight spoilers inop
Roll rate will be reduced and speedbrakes will not
  be available in flight.
Trailing edge flaps normal hydraulic system inop
The trailing edge flaps can be operated with the
  alternate electrical system. Alternate flap
  extension time to flaps 15 is approximately 2
  minutes.
Leading edge flaps and slats normal hydraulic system inop
The leading edge flaps and slats can be extended
  with standby hydraulic pressure. Once extended,
  they can not be retracted.
Main landing gear normal hydraulic system inop
Manual gear extension is needed.
Autobrake inop
Ground spoilers inop
Landing distance will be increased.
Normal and alternate brakes inop
Inboard and outboard brakes have accumulator
  pressure only. On landing, apply steady brake
  pressure without modulating the brakes.
Nose wheel steering inop
Do not attempt to taxi the airplane after stopping.

(Boeing 737 NG QRH 13.12 Manual Reversion)

(Boeing 737 NG FCOM 9.20.24 - Flight Controls - System Description)

Well, the instruments being dead is only an issue in IMC -- if you are in severe clear, then the loss of instrument power is about the least of your concerns

Much of the answer addresses loss of hydraulics.  Even with engines windmilling (which I'm not sure about as a necessary result of the dead EEC's, but we can go with that), you'd still get the engine driven pumps providing some amount of pressure.  Gear & flap extension would be SLOW, but if anything prevents it, I don't think a lack of hydraulic pressure would be it.  OTOH, how many fail-safes are in place that need at least some electrons to be removed,I can't tell offhand.

@UnrecognizedFallingObject You're right that loss of instruments is more of an issue in IMC, but even in VMC it will be difficult to maintain a correct drift down speed without any airspeed indication. The QRH contains tables with pitch values to fly at to maintain various speeds, but without the artificial horizon it will be hard to determine pitch with such accuracy. It's probably doable by just looking at the real horizon, but much more difficult.

@RalphJ Good point, I hadn't considered hydraulic pressure from windmilling engines. I think that would help with the flight controls, but AFAIK both the gear and flap extension will go through some electric control unit (e.g. FSEU, the Flaps/Slats Electronics Unit), so I'm not sure if that could still function.

Manual extension of the gear works fine, including locking into place. I agree that the unpowered FSEU might prevent flap extension, although I don't know for certain. Does it only engage brakes, or does it need to actively remove an inhibit to movement?

@RalphJ I added the relevant diagram from the FCOM. I think, it might be possible without the FSEU powered, but the FCOM is not detailed enough to actually say. The FSEU can for sure move the flaps back for load relief, but I'm not sure if it is required for normal movement. I guess we would need a Boeing engineer to answer that ;)


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How do A320 pilots trim?
So, I am very familiar with the 737NG, and there is a trim switch on the yoke for vertical trim. However, I can't seem to locate this trim switch in the A320. Is there a trim switch on the joystick in an A320? If not, how do they trim?

You can see on videos that when the pilot activate the switch, the trim wheel turn. On both B737 and A320 the trim wheel is located between pilot's and turn and can be turned manually by pilots as a backup way to trim the aircraft (i.e. to move the THS)

@ManuH The point is that there is no such switch on Airbus aircraft. Turning the wheel by hand is the only manual way to change pitch trim in an A320.

@Bianfable the comment was to highlight that both type have a common way to (manually) trim the aircraft. The OP seems to ignore this way exists on the B737.

The pilots do not manually trim an Airbus in pitch. The flight control computers provide automatic trim:

The normal-law flight mode is a load-factor-demand mode with automatic trim and protection throughout the flight envelope.
Following normal law, the sidestick controllers set the elevators and THS to maintain load factor proportional to stick deflection and independent of speed.
With the sidestick at neutral, wings level, the system maintains 1 g in pitch (corrected for pitch attitude), and there is no need for the pilot to trim by changing speed or configuration. Pitch trim is automatic both in manual mode and when the autopilot is engaged.

(A320 FCOM - Flight Controls - Description - Normal Law - Flight Mode)
There is however a mechanical backup:

The pilots use mechanically interconnected handwheels on each side of the center pedestal to control the trimmable horizontal stabilizer.

(A320 FCOM - Flight Controls - Description - Cockpit controls)
You can see the full flight control logic in the following image. The pitch trim is indicated by THS (Trimmable Horizontal Stabilizer):

(image source: A320 FCOM - Flight Controls)

I don't understand half of the diagram, but my guess is that essentially, the computer system on the A320 calculates where the pilot wants the nose at, and trims it for them

@lpydawa Something like that: the pilot controls the target load factor with the sidestick, the computer then calculates the required pitch angle to reach that load factor and then actuates elevators and THS accordingly.

A key to the other abbreviations in this diagram would be useful: e.g. FAC=Flight Augmentation Computers; ELAC=Elevator Aileron Computers; SEC=Spoiler Elevator Computers. FCDC=Flight Control Data Concentrators; ...

@Bianfable that sounds kind of dangerous.... Imagine the computer fails, calculates wrongly, and the whole plane dives or climbs and stalls

@lpydawa You could say the same about the entire flight control logic of Airbus planes, but how often do you hear about an Airbus just randomly crashing? There is more than one of each of these computers and they are constantly cross-checking each other. Have a look at How dissimilar are redundant flight control computers?

@Bianfable so complicated too! No thank you! Sticking with ma trusty and safe 737... Oh wait- MCAS screwed up...

Autotrim :D or Manual trim by moving the trim wheel. Trim wheel has priority over Autotrim. But if you move the wheel with AP on, it will disengage. Also AP doesn't need to be on for Autotrim to work. It's always working in normal and alternate law, until bank angle of 33 Deg, then you are on your own.


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Is the FlightGear sim accurate in 2018, specifically Cessna 172 aircraft?
So, I fly on FlightGear as I like the fact its free. But is it accurate enough to train to fly the real Cessna 172? Are there any better free flight sims? Please give your honest opinion and please try FlightGear before answering my question, I can't deal with people telling me their opinions when they have not tried FlightGear yet.

Every flight sim has its uses and drawbacks. There really isn't anything like climbing into a real cockpit and taking to the skies. You get peripheral vision, motion, you can feel the airplane, the wind, the bumps, etc. You can use a flight sim to learn about what things do and a bit about how things react, but to learn how to use them in reality, you will need to climb into a cockpit and go flying.

Learning to actually fly with a computer simulator would work about as well as learning to ride a bike with a bike simulator.

When you can do a proper walk-around on a real aircraft, you've earned the right to fly it. Unfortunately the only way to learn it is to do it for real. E.g. climbing to the top to open the fuel cap, getting fuel samples, checking for movements of the control surfaces, inspecting the tire etc. Or even as simple as removing the ropes that secure the plane to the ramp, then pulling it out a few meters.

See Can Microsoft Flight Simulator help me learn to fly (or make me a better pilot)? It's not about the same software, but lots of the points made in answers apply just as well to any PC flight simulator.

@kevin Recent versions of Flightgear actually include a virtual walk-around of sorts. If you follow the included checklist for the Flightgear C172, you're even supposed to do things like make sure the seatbelts are secure, let alone the more obvious things like check control surfaces for freedom of movement and correctness, or check the oil pressure on engine start. (Or you can just use the autostart and not bother with the checklist...)

It completely comes down to what you want to train in the sim. I have used FlightGear in my beginnings of when I started my interest into aviation. I guess you are using the default C172. What you can do in this sim is learning to navigate, use the instruments (at least the ones that are modelled to work correctly) and maybe learn checklists. But that's it actually.
If you really want to train for a real Cessna 172 aircraft, I can highly recommend getting Prepar3D and the C172 by A2ASimulations. It's one of the best flight simulation addons in the flight sim industry at this time. A full simulation of everything from oil viscosity to water contamination of fuel. Even a real C172 walk-around is implemented (with fuel inspection, tire inspection, everything) and a full damage model. You have to treat it like in real life to not damage parts. And you can also install a GTN750, GTN650 or GNS systems in her, if you own one of that beauties in real life. Also study level of course.
What you can't train in flight sims, however, is the hand flying.
I own all those addons and can't stop recommending it.

Let us continue this discussion in chat.


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Were Airbus planes always fly-by-wire?
So, as most of you know, Airbus has adopted fly-by-wire (fbw) technology. Pretty much every single plane made by Airbus is a fbw plane. So I was wondering, before computers were everywhere, did Airbus implement fbw systems?

As Ron notes in the comments the A300 did not have it and neither did the A310 however the A300-600 and A310 did have electronically augmented controls according to the airbus site.

One of the A300-600 and A310’s notable innovations had been the
  introduction of electrical signalling on secondary flight controls,
  replacing the web of cables and pulleys tradionally used. Béteille
  wanted to take this evolution further with the next Airbus aircraft –
  to computer-driven digital “fly-by-wire”, in which the deflections of
  the flying control surfaces on the wing and tail are no longer driven
  directly by the pilots’ controls, but by a computer which calculates
  exactly which control surface deflections are needed to make the
  aircraft respond as the pilot wishes.

The A320 on had Fly-By-Wire
There is a nice little video on the same page that has some facts abut Fly-By-Wire tech over the years. A quick history note is applicable here: what we know of today as Airbus had heavy ties to the Concorde program which implemented the first commercially used analog fly by wire system and depending on how you look at it Airbus' first Fly-By-Wire system as well. These engineers who would later go on to work at what is now Airbus and drive the mentality into the designs which is why they have largely chosen it on all their airframes.

Actually Airbus's first full authority FBW airplane was the A-320 in the late 80s.  Their earlier designs used FBW for secondary controls as did lots of other manufacturers.
The Avro Arrow had an analog FBW system, with mechanical reversion, in the 50s.
But to drift off topic a bit, the real granddaddy of all operational aviation "FBW" systems is on the B-29 bomber... although, OK, it wasn't actually a flight control system.  The B-29's General Electric defensive fire control system used a central computer that received analog electrical signals from signal generators called "Selsyns" on each gunner's sighting head, that told the computer where the sight was pointed and the range to the target (by the gunner framing his target with the sight reticle, with the target's wingspan set on the sight, as he tracked it), and the computer corrected for a bunch of factors, parellax between sight and remote turret, TAS, density alt, 50 cal ballistics, computed lead angle etc, and the COMPUTER aimed the guns electrically by driving the remote turrets and pointing the guns where the computer wanted them, not where the gunner was pointing.
In effect, the gunner was telling the computer "I'll point at the target and tell you the range and you work out a firing solution and figure out where to point the guns". Incredibly sophisticated for the early 40s and pretty much the same as what a flight control FBW system does.

There were a number of analog computers in use; the B-29 system was very sophisticated but far from alone in this regard. You might as well use the BMW control system for the BMW 801 as an example (which predates the B-29). The first FBW aircraft in commercial operation was the Concorde - maybe this would be a better fit here.

That is a mechanical controller - mechanical inputs, and mechanical outputs.   A better comparison might have been the Norden bomb sight.  In any case, the GE fire control system was electric commands down wires to a central computer with a mix of electrical and mechanical internals, and the computer electrically controlled the output, which varied from the input based on the computer's "programming".  Basically the essence of a FBW control system where the control surface movement was often different from the input (as opposed to "control by wire" where the input and output are the same).


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How do the VNAV and LNAV work on the 737NG?
So, I'm very confused. Im from a Cessna background, no FMC. So, based on my understanding, you could run the autopilot independent of the FMC right? With Speed mode, HDG SEL mode, and V/S mode, am I right or wrong?
And if you enable VNAV and LNAV, the FMC takes over right? But I heard people say you can change the V/S in VNAV mode. How is that possible? I thought the FMC has full authority, and the flight route you filed in the FMC is final?  And does this mean you can also change the HDG in LNAV mode?

I see some confusion about the Autopilot system here. First let's clear something up:

The B737 has 3 guidance: speed, horizontal and vertical. These are displayed on the top of the PFD from left to right.
Engaging Flight Director displays the guidance but not to follow it, engaging the Autopilot tells the plane to follow the guidance.
Each guidance has several modes. E.g. Possible horizontal guidance modes include "HDG SEL", "HDG HLD", "LNAV" etc. Possible vertical guidance modes include "V/S", "ALT HLD", "VNAV" etc.

LNAV is just one of the horizontal guidance modes. The FMC decides what is LNAV. If LNAV is not engaged, then the FMC does not take part in horizontal navigation.
Your confusion likely arises from armed modes. The B737 allows you to select one active mode, plus one armed mode. For example, consider the LNAV path in FMC is heading North (360 degrees). The plane is to the East of the intended path (right side). By doing the following:

Set the heading window to "340".
Set "HDL SEL" as active mode.
Set "LNAV" as armed mode.

The plane is on a heading to join the LNAV course. When the plane is close*1, the horizontal mode will automatically change from "HDG SEL" to "LNAV".
Similarly, "V/S" can be used to capture VNAV profile when the plane is above or below the intended altitude.
*1: The plane has to be within a certain nautical miles and the intercept angle cannot exceed a certain degree. I will update the exact number when I get reference to the B737 manuals.

What do you mean by "capture the VNAV profile?"

@lpydawa you might want to ask another question on the site for that.

@Ipydawa, LNAV and VNAV are FMS computed mode they correspond respectively to the lateral path( FMS route) and the vertical path ( vertical restrictions you can check on the CDU). Assume you are at cruise but not respecting the FMS cruise altitude, but nevertheless you want to follow the FMS descent profile respecting all restrictions, you will arm the VNAV mode, once you cross the FMS descent profile VNAV will get active and the aircraft will start descending if the A P is active. VNAV and LNAV require to have at least an active FD